This invention, which is a result of a contract with the U.S. Department of Energy, relates generally to the art of microwave heating and more specifically to sintering of ceramic articles using microwave heating.
Although microwave heating is now well known and accepted in the private sector with the development and acceptance of relatively low cost microwave ovens, the adoption of microwave power and development of equipment for use in the industrial sector has been slow. One area in which the use of microwave energy may be applied is in the development and processing of advanced high-temperature ceramics. The sintering of ceramics by microwave energy has some distinct advantages over sintering by conventional gas, electric or oil furnaces or kilns. The advantages of microwave sintering derive primarily from the fact that the heating rate is so much faster than with conventional furnaces. The faster heating occurs because the ceramic is heated directly through interaction of the microwave energy with the ceramic material as opposed to the slow process of radiant or convective heating in a conventional furnace. Increased heating rates usually result in improved densification behavior and rates. A high heating rate allows little time for deleterious effects such as particle-particle neck formation and secondary recrystallization (exaggerated grain growth) to occur before densification. Further, rapid heating can also reduce by as much as 10-15 percent the ultimate temperature necessary to achieve full density. Such conditions can lead to the attainment of dense ceramic articles with very fine grain sizes which is a key feature in producing high strength advanced ceramics.
With microwave heating, the rate at which a particular ceramic material may be heated is related to its dielectric loss factor (which is the product of the materials specific dielectric constant and the tangent of its dielectric loss angle) and the incident amount of microwave energy. Unfortunately, in present known applications of microwave sintering of ceramic materials, the article size is limited to small samples (approximately 10 cm.sup.3) because of nonuniform heating. Many of the earlier microwave heating experiments used tuned single mode resonant cavities with a high Q factor. These cavities yield very high electric field intensities and high heating efficiencies over small volumes. Thereby, limiting the size of ceramic articles which can be sintered.
Other experiments have been carried out in non-resonant or untuned cavities. However, these also suffered from nonuniform heating due to the small number of standing wave patterns which were available for coupling with the ceramic material. The number of standing wave patterns or modes N which are available is related to a characteristic dimension of the cavity d times the frequency of operation f quantity squared. EQU N.varies.(df).sup.2
As N gets larger, the electric field intensity in the cavity becomes more and more uniform. Therefore, in order to achieve uniformity either the cavity size or the frequency of operation must increase.
Although the above relationship is generally known in the art of microwave oven design, research in the art of microwave sintering has not considered the possibilities of using higher frequency and/or enlarged microwave cavities. Up until now microwave sintering of ceramics has been performed with small cavities operating at 2.45 GHz with a few kilowatts of cw power. Since the loss tangent of most ceramic materials is low in the microwave frequency range and increases as the frequency is increased, there is a need for a microwave sintering system which is capable of applying microwave power to articles to be sintered at substantially higher frequencies than 2.45 GHz. Further, the power (P) absorbed by the article follows the relationship: EQU P.varies.f.epsilon..sub.R tan.delta. .vertline.E.vertline..sup.2,
where f is the frequency of the microwave energy, .epsilon..sub.R is the relative dielectric constant, tan.delta. is the dielectric loss tangent and .vertline.E.vertline..sup.2 is the electric field intensity squared or the energy density. Therefore, not only is there a need to provide a system in which the sintering frequency may be increased, there is a need for a system in which the power level of the applied microwave energy may be increased as well so that these two factors may be combined in a modest size cavity to reduce the sintering time and improve the electric field uniformity within the cavity. Such a system will not only allow uniform sintering of larger ceramic articles, but will also allow non-uniform shaped articles to be sintered as well with improved uniform mechanical strength. By making the cavity dimensions much, much greater (at least 100 times) than a free space wavelength of the applied microwaves, the power density in the cavity may be made uniform by allowing the microwave energy to reflect off the cavity walls many times, thus exciting a large number of cavity modes. This operating domain is not practical at 2.45 GHz because the cavity size would become prohibitively large, thereby making it virtually impossible to meet the power density requirements.